We are working with medicinal chemists to design inhibitors of clinically-troublesome bacterial -lactamases, which provide resistance to penicillins and cephalosporins. By working at cryogenic temperatures, we hope to stabilize and visualize reaction intermediates in the inhibition pathway. Four -lactam complexes of two class C -lactamases from Enterobacter cloacae (P99 and GC1) were examined at 100-K on beamline A1 with the 2k Princeton detector in binned mode. Aztreonam and ampicillin were complexed with the GC1 enzyme, and potential inhibitors KM233 and DVR3 were complexed with the P99 enzyme. The P99 and GC1 complexes gave data to about 2.7 and 2.3 resolution, respectively. Initial phasing will be based on the known native structures. Maps of the GC1 complexes are now being examined. New crystals of the plasmid-mediated SHV class A -lactamase were characterized. The small crystals (25x75 microns) diffracted to 2.4 resolution. A poorly resolved reciprocal spacing on the Princeton detector, estimated to be about 250 -1, has hindered indexing by DENZO so that XGEN will be used instead. Because there appear to at least 4 copies of the molecule per asymmetric unit, we may not continue to use this crystal form. In the remaining time, new 50 micron crystals of vanB, an aminoacid ligase providing vancomycin resistance to enterococci, were quickly characterized in order to plan for future work and were found to diffract to about 4 , for a monoclinic cell with 85x83x83 and =99-.